The present invention relates to a microbiological testing apparatus and an associated method. More specifically, the present invention relates to an apparatus for use in the automated antibiotic susceptibility testing of samples, such as those from patients possibly infected by a microorganism.
Agar disk diffusion is a widely recognized microbiological assay for measuring susceptibilityxe2x80x94a parameter effectively defined by the assay itself. The susceptibility of a microorganism to a given antibiotic is essentially a description of the size of the inhibitory zone resulting from placement of a permeable disk impregnated with the given antibiotic onto an agar surface inoculated with a sample culture of the microorganism. This parameter provides a measure of the ability of the antibiotic compound to stem growth of the target culture, but it is also a complex function of diffusion constants and other kinetic factors.
Early laboratory standards for the agar diffusion assay involved qualitative evaluation by a laboratory technician, characterizing the tested bacterium""s interaction with the antimicrobial agent as xe2x80x9csusceptiblexe2x80x9d, xe2x80x9cmoderately susceptiblexe2x80x9d, xe2x80x9cintermediatexe2x80x9d or xe2x80x9cresistantxe2x80x9d, depending on the size of the inhibition zone surrounding the antibiotic impregnated disk.
Of additional use to the clinician is a related quantitative measure of susceptibility, known as xe2x80x9cminimum inhibitory concentrationxe2x80x9d (MIC). Although still requiring additional information to translate the parameter into a prescription for clinical practice, this quantitative measure eliminates some sources of complexity and uncertainty relative to qualitative susceptibility. An additional useful clinical parameter is the xe2x80x9cinhibitory quotientxe2x80x9d, which expresses the ratio of the drug concentration in a particular body tissue at a lowest clinical dose to the minimum inhibitory concentration.
The MIC is ideally determined by an assay appropriately called the dilution method, which straightforwardly involves inoculating a series of test tubes with the target culture, the test tubes containing a series of dilutions of the target antibiotic. One series of test tubes therefore tests only one culture and one antibiotic, in contradistinction to an agar diffusion assay petri dish, which may test a plurality of antibiotics simultaneously with less material and expense. The advantage of the dilution method is that it provides less ambiguously interpretable quantitative results relative to the agar diffusion method, while its disadvantage is primarily its expense, both in materials and labor.
It is therefore desirable to have a device which automatically translates a dimension of an inhibition zone on an agar diffusion assay plate into a more clinically useful quantitative measure of drug-bacterium interaction, such as the MIC. Such a device is disclosed by U.S. Pat. No. 4,701,850. It is further desirable to have a device which automates the process of reading the apposite linear dimension of the inhibition zone, such devices being revealed in subsequent United States patents. The relation of the diameter of the inhibition zone to the MIC for an unknown biological agent is approximated by a linear relation, the parameters for which assumed relation for a particular antibiotic being determined by statistical estimation based on the scatter of data points whose coordinates are inhibition zone diameters and actual minimum inhibitory concentrations determined by dilution assay for a particular microorganism, the relation being assumed linear and being assumed to persist for untested organisms.
Since a number of different antibiotics are simultaneously tested against an unknown culture on a single agar plate and since these antibiotics are characterized by different values of the linear parameters relating inhibition zone dimensions to estimated MIC, and by differing values of the measured dimension of the inhibition zone in a given test, it would be advantageous to have a method of associating the zone surrounding a given antibiotic disk with the subject antibiotic compound without further operator intervention or opportunity for human error. Such a method is disclosed in U.S. Pat. No. 6,107,054.
In extant microbiological testing devices which measure the diameter of microbe inhibition zones about drug diffusion disks, illumination of the petri plate and the disks thereon is by a light source separate from the camera or other optical detection device. A switch must be manually thrown prior to image acquisition. In addition, it is desirable to de-energize the illumination afterimage acquisition.
An object of the present invention is to provide an improved apparatus and/or associated method for microbiological testing.
A further object of the present invention is to provide a microbiological testing apparatus and/or method which enables automatic determination of a susceptibility of a microorganism to an antibiotic agent.
An additional object of the present invention is to provide such a microbiological testing apparatus and/or method which is easy to use.
It is a supplemental object of the present invention to provide such a microbiological testing apparatus and/or method which is inexpensive to use.
A particular object of the present invention is to provide an essentially automated microbiological testing apparatus and/or method wherein a dimension of an inhibition zone associated with an antibiotic impregnated disk on an agar plate is measured automatically.
A more particular object of the present invention is to provide such a microbiological testing apparatus and/or method which facilitates association of a microorganism susceptibility measurement with a particular antibiotic agent.
These and other objects of the present invention will be apparent from the descriptions and illustrations provided herein.
The present invention is directed in part to an apparatus for use in carrying out a microbiological assay on samples in a petri dish. The petri dish or plate contains a nutrient medium, a plurality of drug diffusion disks each disposed on the medium and carrying an antibiotic agent, and a microbial solution deposited on the medium. The apparatus of the present invention comprises a support for holding the petri plate, an optical monitoring device, and a plurality of light emitting diodes. The optical monitoring device optically detects microbe-growth inhibition zones arising about the diffusion disks and is disposed above the support. The optical monitoring device is an imaging device such as a video camera, a digital camera, a scanner, a linear array, etc. The diodes are disposed in an array above the support and below the optical monitoring device for illuminating the nutrient medium and the drug diffusion disks.
Preferably, the diodes are diffuse-white-light emitting diodes. This kind of diode provides a broad-spectrum illumination adequately approximating natural light. Pursuant to the invention, the number and distribution of the diodes results in a sufficient degree of illumination for purposes of determining the sizes of microbe-inhibition zones about the drug diffusion disks and also for purposes of automatically reading the antibiotic identification codes imprinted on the disks. A microbiological testing apparatus with an automated reading of antibiotic identification codes is disclosed in U.S. Pat. No. 6,107,054, the disclosure of which is hereby incorporated by reference.
In accordance with another feature of the present invention, electrical transmission components are provided for supplying electrical power to the diodes from a computer in response to an activation of the optical monitoring device. The electrical transmission components may include a relay connected at an output to the diodes, connected at a control input at least indirectly to the optical monitoring device, and connectable at a power input to the computer. Electrical power may also be provided to the optical monitoring device from the computer. In that case, an electrical lead is connected at one end to the optical monitoring device and connectable at an opposite end to the computer for supplying power from the computer to the optical monitoring device.
The optical monitoring device is typically connectable to the computer for transmitting thereto image data for processing.
In accordance with further features of the present invention, the diodes are disposed in an annular array, for instance, on an upright annular surface, while an annular light shield is disposed between the diodes and the optical monitoring device. Preferably, the annular surface is white, thereby enhancing the amount of illumination falling on the petri plate and the drug diffusion disks from the diodes.
Pursuant to another embodiment of the present invention, an apparatus for carrying out a microbiological assay comprises a support for holding a petri plate containing a nutrient medium, a plurality of drug diffusion disks each disposed on the medium and carrying an antibiotic agent, and a microbial solution deposited on the medium. The apparatus additionally comprises an optical monitoring device disposed above the support for optically detecting microbe-growth inhibition zones arising about the diffusion disks. The apparatus further comprises a plurality of diffuse-white-light emitting diodes disposed in an array above the support and below the optical monitoring device for illuminating the nutrient medium and the drug diffusion disks. The apparatus also comprises a computer connected to the optical monitoring device for receiving and processing image data therefrom, electrical transmission circuitry being operatively connected to the computer, the optical monitoring device and the diodes for transmitting power from the computer to the diodes upon an activation of the optical monitoring device.
As discussed above, the electrical transmission circuitry may include a relay connected at an output to the diodes, at a control input at least indirectly to the optical monitoring device, and at a power input to the computer. The computer is also connected to the optical monitoring device for supplying power to the optical monitoring device.
In a particular embodiment of the invention, an apparatus for carrying out a microbiological assay comprises (a) a support for holding a microbiological assay petri plate, (b) a camera disposed above the support for optically detecting microbe-growth inhibition zones arising about diffusion disks on the petri plate, (c) a plurality of diffuse-white-light emitting diodes disposed in an array above the support and below the optical monitoring device, (d) a light shield disposed between the diodes and the camera for blocking a passage of light directly from the diodes to a lens or aperture of the camera, (e) a relay or switch connected to the diodes and at least indirectly to the camera for enabling a transmission of electrical power to the diodes in response to a signal from the camera, (f) a first coupling element for enabling information transfer between the camera and a computer, and (g) a second coupling enabling power transfer from the computer to the camera and the relay.
A method for use in carrying out a microbiological assay, in accordance with the present invention, analyzes a petri plate containing a nutrient medium, a plurality of drug diffusion disks each disposed on the medium and carrying an antibiotic agent, and a microbial solution deposited on the medium. The method includes activating an optical monitoring device disposed above the petri plate for optically detecting microbe-growth inhibition zones arising about the diffusion disks, and energizing a plurality of light emitting diodes disposed in an array above the petri plate and below the optical monitoring device to illuminate the nutrient medium and the drug diffusion disks.
Preferably, the energizing of the diodes includes transmitting power from a computer to the diodes. In that event, the energizing of the diodes may be carried out automatically upon the activating of the optical monitoring device. In addition, image data is transmitted from the optical monitoring device to the computer and the computer is operated to process the image data. The energizing of the diodes may additionally include activating a relay or switch to transmit power from the computer to the diodes.
A microbiological testing apparatus with a lighting array in accordance with the present invention provides several major and novel advantages over existing lighting. First, the diodes consume very little power and thus enable one to power the illumination assembly via a PC interface, eliminating the need for a xe2x80x9cwall/externalxe2x80x9d power supply. Second, the diodes provide an intense, broad-spectrum evenly distributed light, eliminating the need for a light diffuser and enabling good color analysis. Third, the lighting assembly is very rugged and relatively unbreakable (a problem during shipping). Fourth, the lighting assembly has an extremely long life, eliminating the need to replace it during the expected product life of 5 to 10 years. Fifth, the lightning assembly can be switched on and off automatically using the camera controls at the PC; this eliminates the requirement for external light on-off switches and/or timer switches activated by placement of test-plates. All these advantages provide improvements to performance of the image-analysis, reduce/eliminate maintenance, simplify construction and tend to reduce costs.